1. Field of the Invention
The present invention relates to audio transmitting and receiving systems, and more particularly to such a system which can be effectively applied to automatic braodcasting systems and the like.
2. Description of the Prior Art
Society has become information-oriented to even greater extent with the progress of information technology, and it has become possible to offer automatically various kinds of services to the public without the aid of any human hands. In railway stations, for example, an automatic broadcasting system has been extensively employed which is combined with an automatic control system, such as an automatic train control system, so as to automatically broadcast the departure and arrival of trains.
An audio response unit employed in such an automatic broadcasting system is generally installed in, for example, a central control room, and audio signal outputs of the audio response unit are transmitted to individual railway stations by cables or equivalent transmission means. However, the audio signals are subject to more attenuation with the increase in the transmission distance, and noises tend to be mixed in the audio signals being transmitted thereby greatly deteriorating the quality of the audio signals reproduced and amplified at the receiving end.
In an effort to obviate these drawbacks, a method has been proposed in which an analog audio signal output of an audio response unit is converted into a PCM (Pulse Code Modulation) signal by a pulse code modulating terminal unit to be transmitted to the receiving end by means such as a cable, and the PCM signal is demodulated into an analog signal at the receiving end. In this method, audio signals are first recorded in analog form on a recording drum in the audio response unit, and the recorded audio signals are then continuously read out from the recording drum and applied to a time division switch. Instructions are applied from control means such as a control computer, to the time division switch, so that the audio signals are converted into PAM (Pulse Amplitude Modulation) signals to edit the messages to be alloted to the individual channels. The converted signal is called a channel multiplex PAM signal. PAM signals corresponding to the individual channels of the channel multiplex signal are separated by channel separation pulses and then passed through demodulators to be converted into analog audio signals. In the case of applications to broadcasting and the like services, arrangement is made so that interruption of additional audio information can be made in each channel. To this end, a control console is provided to assign the interrupted channel, and the interrupting audio information is applied to an analog mixer together with the analog audio signal to be mixed with the latter. The analog signal output of the analog mixer is transmitted after being passed through a suitable PCM means since it is necessary to convert the analog signal into a PCM signal suitable for the long-distance transmission. This method is however defective in that the necessity for special provision of the additional PCM means leads to an increase in the number of system components resulting in further complexity of the structure.